Support optional URI elements

[rhernandez35]

Currently, the Rust code generator assumes that all members with the httpLabel trait are required. This isn't always true. For example, S3's GetObjectRequest structure binds partNumber to a URI element, but it's necessarily optional since it's only accepted when retrieving a single part of a multi-part object. As is, the generated code forces this field to be specified in all requests.

I'd say "just examine the member for the required trait," but I sadly don't see many models using it. FWIW, The AWS service models bundled with botocore appear to correctly model all required members regardless of where they appear on the wire. Are these service definitions hand-maintained?

[rcoh]

Interesting. The smithy spec mandates that things marked as httpLabel are required:

httpLabel members MUST be marked as required trait.

From https://awslabs.github.io/smithy/spec/http.html#httplabel-trait

Part number is actually in httpQuery:

                "PartNumber": {
                    "target": "com.amazonaws.s3#PartNumber",
                    "traits": {
                        "smithy.api#documentation": "<p>Part number of the object being read. This is a positive integer between 1 and 10,000.\n         Effectively performs a 'ranged' GET request for the part specified. Useful for downloading\n         just a part of an object.</p>",
                        "smithy.api#httpQuery": "partNumber"
                    }
                },

Is that modeled correctly?

[rcoh]

The underlying field is unboxed which is why it isn't modeled as Option<u32>

[rcoh]

The actual issue here is that the Http protocol binding traits send 0 in the query param when the field is unset. Although this behavior is technically within spec, in the case of GetObject (as well as many other APIs) sending 0 is actually an error.

[rcoh]

Closed and opened #108